Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5625_Библиотеки_им_академика_М_И_Перельмана

PSs activate B cells by crosslinking approximately 15–20 Bcell receptors (BCRs), triggering a series of protein phosphorylation steps which leads to an increase in free intracellular calcium (Figure1.1a)[56]. The need to crosslink multiple BCRs to achieve Bcell activation is noteworthy, as it explains the reason why the immuno­genicity of PS vaccines is sizedependent, with only highmolecularweight antigens capable of inducing an effective immune response. Following stimulatory cytokines or costimulatory signals expressed by other cells of the immune system[57–59], B cells are finally activated. They mature into plasma cells and secrete antibodies

(mainly IgM) but since the entire process takes place without the direct involvement of T cells, the development of MBCs and the induction of immunological memory do not occur.

Unlike PSs, proteins and peptides are instead Tcelldependent (TD) antigens, as they stimulate helper T lymphocytes to elicit an immune response leading to spe­cific antibody response. TD antigens are immunogenic even in early childhood, the immune response induced can be boosted and enhanced by adjuvants, and it is characterized by an antibody class switch with the production of highaffinity and antigenspecific IgG. The haptencarrier concept[60] underpinning the develop­ment of glycoconjugate vaccines is based on the fact that the covalently linked pro­tein carrier confers Tcelldependent properties to the glycoconjugate, which is eventually able to produce carbohydratespecific MBCs. The molecular mechanism of the immune response to glycoconjugate vaccines has been thoroughly investi­gated[61, 62], albeit some steps are still unclear. After administration, the conjugate is taken up by antigenpresenting cells (APCs), mainly dendritic cells (DCs), but also macrophages and B cells can play the same role. Engulfment by APCs is pro­moted by stimulation of pathogen recognition receptors (PRRs), a large family of receptors expressed on APCs surface able to respond to a huge variety of pathogen associated molecular patterns (PAMPs), structurally and chemically diverse com­pounds highly conserved in pathogens but absent in their multicellular host. PRRs stimulation creates the necessary proinflammatory context (expression of costimu­latory molecules and secretion of soluble cytokines and chemokines), leading to full maturation of DCs, antigen uptake, and intracellular processing (Figure 1.1b). In particular, the glycoconjugate antigen is chopped through the intracellular endoso­mal compartments. While the saccharide portion is depolymerized by oxidative agents (reactive oxygen species, ROS, and reactive nitrogen species, RNS)[63, 64], the protein portion is processed by proteases into small peptides. Within a few days, mature DCs reach the draining lymph nodes, where peptide antigens are conveyed to the cell surface in association with the major histocompatibility complex class II (MHCII) protein to be presented to T lymphocytes via interaction with the Tcell receptor (TCR). T lymphocytes are then activated as helper T cells (Th or CD4 cells), which provide appropriate stimulatory signals to elicit a conventional TD immune response. Th cells prime the maturation process of resting B cells, driving their proliferation and differentiation into antibodysecreting (mainly IgM) plasma cells and MBCs, ensuring the establishment of the immunological memory. Contrary to plasma cells, MBCs survive for a long time in the body and respond rapidly to subsequent exposures of the same antigen by secreting highaffinity IgG antibodies.

contain and present the antigen to the B cells. The formation of GCs is an essential step during the immune response to an infection or after vaccination, since the affinity maturation process and the class switch from IgM to IgG occur in the GCs, specifically in two distinct regions called the light and dark zones.

amidine bonds. All these homobifunctional linkers are soluble in water and ensure highly specific and easy conjugation protocols that have been exploited in the prepara­tion of different glycoconjugate vaccines[101-103]. Diethyl squarate also reacts with amino groups on the sugar and the protein to give crosslinked coupling products[104]. Halogen bearing homofunctional spacers, such as dibromomaleimide or 3,3 bis(bromomethyl)oxetane, are also available (Figure1.2). They are characterized by a cyclic scaffold and by the presence of two halogens that can be displaced in a twostep